Fix inconsistent results with presence in the sample

[glennblock]

The behavior exhibited is that one socket.io instance does not see the list of connections from the other socket.io instance. In these cases the other instantly normally IS showing the full list.

Retry logic requesting the list of connected clients MAY solve this problem.

Dev Estimate: 8
Test Estimate: 3

[christav]

Noticed something while testing something else - store takes a little while to start up. Could we be trying to send & receive presence information before the store is actually ready / running?